1. Field of the Invention
The present invention relates to a light emitting device with a light emitting element that emits fluorescent light or phosphorescent light upon application of electric field to a pair of electrodes of the element which sandwich a layer containing an organic compound, and to a method of manufacturing the light emitting device. In this specification, the term light emitting device includes an image display device, a light emitting device and a light source (including illuminating device). Also, the following modules are included in the definition of the light emitting device: a module obtained by attaching to a light emitting device a connector such as an FPC (flexible printed circuit; terminal portion), a TAB (tape automated bonding) tape, or a TCP (tape carrier package); a module in which a printed wiring board is provided at an end of the TAB tape or the TCP; and a module in which an IC (integrated circuit) is directly mounted to a light emitting element by the COG (chip on glass) system.
2. Description of the Related Art
Light emitting elements, which employ organic compounds as light emitting member and are characterized by their thinness and light weight, fast response, and direct current low voltage driving, are expected to develop into next-generation flat panel displays. Among display devices, ones having light emitting elements arranged to form a matrix shape are considered to be particularly superior to the conventional liquid crystal display devices for their wide viewing angle and excellent visibility.
It is said that light emitting elements emit light through the following mechanism: a voltage is applied between a pair of electrodes that sandwich a layer containing an organic compound, electrons injected, from the cathode and holes injected from the anode are re-combined at the luminescent center of the layer containing the organic compound to form molecular excitons, and the molecular excitons return to the base state while releasing energy to cause the light emitting element to emit light. Known as excitation states are singlet excitation and triplet excitation, and it is considered that luminescence can be conducted through either one of those excitation states.
Such light emitting devices having light emitting elements arranged to form a matrix can employ passive matrix driving (simple matrix light emitting devices), active matrix driving (active matrix light emitting devices), or other driving methods. However, if the pixel density is increased, active matrix light emitting devices in which each pixel (or each dot) has a switch are considered as advantageous because they can be driven with low voltage.
A layer containing an organic compound (strictly speaking, light emitting layer), which is the center of a light emitting element, is classified into low molecular weight materials and polymeric (polymer) materials. Both types of materials are being studied but polymeric materials are the ones that are attracting attention because they are easier to handle and have higher heat resistance than low molecular weight materials.
The conventional active matrix type light emitting device has the structure comprising a light emitting element in which an electrode electrically connected with TFT on a substrate is formed as an anode, a layer containing an organic compound is formed thereon, and cathode is formed thereon. And light generated in the layer containing, the organic compound can be observed at the TFT side through the anode that is a transparent electrode.
There has been a problem in the structure that an opening ratio is restricted depending on an arrangement of TFT and wirings in a pixel portion when definition is to be improved.
Therefore, manufactured in the present invention is an active matrix light emitting device that has a light emitting element with a structure called a top emission structure. In the top emission structure, a TFT side electrode which is electrically connected to a TFT on a substrate serves as an anode (or a cathode), a layer containing an organic compound is formed on the anode (or the cathode), and a cathode (or an anode) that is a transparent electrode is formed on the layer containing an organic compound.
Compared to the bottom emission structure, the number of material layers through which light emitted from the layer containing the organic compound passes is smaller in the top emission structure and stray light caused between material layers of different refractive indexes is accordingly reduced.
Not all of light generated in the layer containing the organic compound are taken out in the direction toward the TFT from the transparent electrode serving as the cathode (or the anode). For example, light emitted in the lateral direction (the direction parallel to the substrate face) is not taken-out and therefore is a loss. An object of the present invention is to provide a light emitting device structured so as to increase the amount of light which is taken out in a certain direction after emitted from a light emitting element, as well as a method of manufacturing this light emitting device.
A problem of the top emission structure is that its transparent electrode has high film resistance. The film resistance becomes even higher when the thickness of the transparent electrode is reduced. When the transparent electrode that serves as a cathode (or an anode) is high in film resistance, a voltage drop makes the intra-plane electric potential distribution uneven and the luminance becomes fluctuated among light emitting elements. Another object of the present invention is therefore to provide a light emitting device structured so as to lower the film resistance of a transparent electrode in a light emitting element, as well as a method of manufacturing the light emitting device. Still another object of the present invention is to provide an electric appliance that uses this light emitting device as its display unit.
In accordance with the present invention, a laminate of metal films and a light absorbing multi-layered film are formed continuously, patterning is conducted to forma first electrode comprising metal films covered with the light absorbing multi-layered film, an insulator covering the end of the first electrode (referred to as bank or partition wall), then etching is conducted in a self-alignment manner using the insulator as a mask and a portion of the insulator is etched and a central portion of the first electrode is etched thinly to form a step at the end. By the etching, the first electrode is formed thinly at a central portion and as a planar surface while the first electrode is formed thick at the portion covered with the insulator, that is, in a concaved shape. A light absorbing multi-layered film is disposed on the portion of the first electrode covered with the insulator to absorb external light. Then, a layer containing an organic compound and a second electrode are formed at least on the central portion of the first electrode, to complete a light emission device.
In the present invention, emitted light in the lateral direction is reflected or collected at an inclined surface formed in the first electrode to increase the amount of emitted light taken out in one direction (direction passing through the second electrode).
Accordingly, the portion to form the inclined surface is preferably made of a light reflecting metal, for example, a material mainly comprising, for example, aluminum or silver, while a central portion in contact with the layer containing the organic compound is preferably made of an anode material of larger work function, or a cathode material of smaller work function. When the portion to form the inclined surface is made of a material, for example, comprising aluminum or silver as a main ingredient, since it reflects external light, it is preferred that portions other than the inclined surface are covered with a material of low reflectance, preferably, a light absorbing multi-layered film.
The light absorbing multi-layered film may be formed, for example, by laminating a silicon nitride film, a metal nitride film (typically, titanium nitride film, tantalum nitride film, etc.) and a silicon nitride film each at an appropriate film thickness on a metal layer of high reflectance (typically, metal layer comprising aluminum as a main ingredient) and, when external light is incident, it is decreased by optical interference absorption caused between each of the layers. The region where the light absorbing multi-layered film is disposed does not overlap with the emitted light region.
Further, the inclined surface formed in the first electrode prevents emitted light from the light emission device (also including emitted light in the lateral direction) from reaching a TFT.
The constitution 1 of the invention disclosed in the present specification provides;
a light emission element comprising, on a substrate having an insulative surface, a first electrode connected with a thin film transistor and an insulator covering the end of the first electrode and
a layer containing an organic compound in contact with the first electrode and a second electrode in contact with the layer containing the organic compound, in which
the first electrode has an inclined surface, the inclined surface reflects emitted light from the layer containing the organic compound, and
a light absorbing multi-layered film absorbing external light is disposed on the portion of the first electrode covered with the insulator.
The constitution 2 of another invention provides a light emission device including;
a light emission element comprising, on a substrate having an insulative surface, a first electrode connected with a thin film transistor,
an insulator covering the end of the first electrode, and
a layer containing an organic compound in contact with the first electrode and a second electrode in contact with the layer containing the organic compound, in which
the first electrode is formed as a concave shape in which the central portion has a reduced film thickness than that at the end, and a light absorbing multi-layered film for absorbing external, light is disposed on the portion of the first electrode covered with the insulator.
Further, the constitution 3 of a further invention provides a light emission device including;
a light emission element comprising, on a substrate having an insulative surface, a first electrode connected with a thin film transistor,
an insulator covering a portion of the first electrode covered with the insulator, and
a layer containing an organic compound in contact with the first electrode and a second electrode in contact with the layer described above, in which
the first electrode has a multi-layered structure in which the number of laminate at the end is larger than the number of laminate at the central portion in the first electrode, and a light absorbing multi-layered film absorbing external light is disposed on the portion of the first electrode covered with the insulator.
Further, the light absorbing multi-layered film absorbing external light is disposed also on wirings or electrodes formed in one identical step and the constitution 4 of a further invention provides a light emission device, including,
a light emission element comprising, on a substrate having an insulative surface, a first electrode connected with a thin film transistor,
an insulator covering the end of the first electrode, and
a layer containing an organic compound in contact with the first electrode and a second electrode in contact with the layer containing the organic compound, in which
the wirings or the electrode formed on one identical layer with the first electrode are in a multi-layered structure, and the light absorbing multi-layered film absorbing external light is disposed.
Further, in accordance with the invention, the shape of an insulator (referred to bank, partition wall, barrier wall, etc.) disposed between each of pixels is improved for eliminating coverage failure, etc. upon forming an layer containing an organic compound layer comprising a polymeric material by a coating method. In each of the constitutions described above, the upper end of the insulator is provided with a curved surface having a radius of curvature, and the radius of curvature is from 0.2 μm to 3 μm. Further, the tapered angle for the insulator may be 35° to 70°.
By the provision of the radius of curvature; the step coverage is enhanced and a layer containing an organic compound to be formed subsequently can be formed even when it is extremely thin.
Further, in each of the constitutions described-above, the first electrode has an inclined surface directing to the central portion of the first electrode and the angle of inclination (also referred to as a tapered angle) is larger than 50° and smaller than 60° and, more preferably, 54.7°. It is necessary that the angle of inclination, the material and the thickness of the layer containing the organic compound, or the material and the thickness of the second electrode are properly selected such that the light reflected on the inclined surface of the first electrode is not dispersed between layers or does not form stray light.
Increment in the take-out efficiency in a case of adopting the structure of the invention is simulated by the procedures described below.
At first, the effect of the structure according to the invention is estimated by regarding the emission from a layer containing an organic compound as optical rays emitted evenly in all of the directions and considering the same geometrically.
Snell's law (ni·sin θi=nj·sin θj, upon incident at an angle θi from a film with a refractive index ni and transmission at an angle θj to a film with a refractive index ni) and total reflection condition (all optical rays are reflected along the path symmetrical with a normal line upon exceeding the value for θi at θj=90° according to Snell's law (critical angle)) are used as a basic law.
The conditions when light emits from the parallel multi-layered film into atmospheric air are considered as below.
As shown in FIG. 12, when the path of light from a layer containing an organic compound transmitting the parallel multi-layered film and outgoing to atmospheric air is observed, a relation: (nel sin θel=nl·sin θ1= - - - =sin θair) is established according to the law described above. Note that, nel and nl denote reflective indexes of a layer containing an organic compound and the layer on the layer containing the organic compound, respectively. And the reflective index of air is assumed 1. When the θair is less than 90°, θ1 etc. are also less than 90° (θel in case of θair=90° is defined as θc). So long as the light emits to the atmospheric air, it does not reflected totally at the boundary in the inside. Accordingly, the condition of θel under which the light goes to the atmospheric air may be θ<θel<θc.
Referring to the structure shown in FIG. 13, the light outgoing into the atmospheric air includes the following two types of paths. That is, it includes a case not reflecting on the inclined surface but outgoing to the atmospheric air (path 1) and a case of outgoing into atmospheric air by way of reflection on the inclined surface (path 2). In the path 1, since the path of the light is identical with a simple case of transmitting the parallel multi-layered film, the condition outgoing to the atmospheric air is: θ<θel<θc. In the case of the path 2, by re-arranging the inclined surface in the case of path 2 into, the case of FIG. 14, it can be seen that the path 2 is identical with the case of the path 1 in which the incident angle changes from θel to θel±θt. Thus, the condition outgoing into the atmospheric air can be expressed as: θ<θel±θt<θc. However, since θc<θel at the horizontal portion because it has to be totally reflected, it is after all only 0<θel−θt<θc. That is, as shown in FIG. 14, while there are two ways of incidence to the inclined surface for one θel, only one of them can transmit.
Further, the efficiency for taking out emitted light is considered as described below. The ratio of the range of the angle at which the light starting from the vertical intermediate point of the layer containing the organic compound outgoes into the atmospheric air relative to the total solid angle is defined as a take-out efficiency. Since the ratio of the angle of light from θ to θ+dθ can be substituted with an area ratio on a unitary spherical surface having an emission point as the center as shown in FIG. 15, it is (2π sin θdθ)/4π. Accordingly, when the range θ is θ1 to θ2, {2π(−cos θ1+cos θ2)}/4π is defined as a ratio of the light take-out efficiency. Thus, assuming that the entire element has a structure symmetrical with the axis of a normal line, the light take out efficiency can be determined from the permissible range for θel being considered in a two-dimensional manner.
Assuming the refractive index of air as 1 and the refractive index of the layer containing the organic compound as 1.78 (the refractive index of Alq3, which is representative example of the layer containing the organic compound), sin 90°=1.73 sin θc is established according to the Snell's law, and θc=35.31° can be obtained. In the case of path 1, the range for emission angle is: θ<θel<θc in view of θ<θel<θc, and the take out efficiency is 18.4% in view of −cos θ+cos θ=0.184. In a case of path 2, θel>θc is required because the total reflection condition must be met. We obtain θt<θel<θc+θt from 0<θel−θt<θc. When θt<θc, the lower limit for θt<θel<θc+θt changes in which the range for outgoing angle is: θc<θel<θc+θt and the take out efficiency is (−cos(θc+θt)+cos θc)/2. Further, in a case of θc+θt>π/2, the upper limit for θt<θel<θc+θt changes in which the range for outgoing angle is θt<θel<π/2 and the take out efficiency is (−cos π/2+cos πt)/2. In addition, in a case of: (θc<θt<π/2−θc), θt<θel<θc+θt is established as it is in which the range for outgoing angle is θt<θel<θc+θt and the take out efficiency is: −cos(θc+θt)+cos θt)/2. The relation between the increases of light take-out efficiency due to the inclined surface formed in the first electrode, and the tapered angle θt for the inclined surface is introduced as shown in FIG. 2B. In FIG. 2B, peaks for the increases of light take-out efficiency is present at the points of: θt=54.69°. The result in FIG. 2B is a result of simulation not considering the absorption and interference of light in the film, assuming the vertical intermediate point of the layer containing the organic compound as the light emission point.
Further, FIG. 18 shows the reflectance of an aluminum film containing a slight amount of Ti and reflectance of a TiN film (100 nm).
Further, in each of the constitutions described above, the second electrode is a light transmitting conductive film, for example, a thin metal film, a transparent conductive film, or a laminate thereof.
In each of the constitutions described above, the first electrode has a concaved shape and the concaved shape is formed in a self-alignment manner using the insulator as a mask. Accordingly, there is no increase of the mask, in view of the formation of the first electrode shape. The end portion of the concaved shape (upper end of the inclined surface) substantially agrees with the lateral side of the insulator and, in view of the step coverage, the angle of inclination on the inclined surface of the first electrode is desirably identical with the angle of inclination on the lateral side of the insulator.
In each of the constitutions, the first electrode is an anode and the second electrode is a cathode. Alternatively, in each of the constitutions described above, the first electrode is a cathode and the second electrode is an anode.
Further, in each of the constitutions described above, the light absorbing multi-layered film formed on the portion of the first electrode covered with the insulator contains at least one layer of a light transmitting nitride insulation film. Specifically, the light absorbing multi-layered film disposed on the first electrode at least has a three-layered structure comprising a light transmitting film, a film partially absorbing light and a light transmitting film, in which the light transmitting film contains at least one layer comprising Al2O3, SiO2, ZrO2, HfO2, Sc2O3, TiO2, ITO or ZnO.
Alternatively, in each of the constitutions described above, the light absorbing multi-layered film formed on the first electrode may be a light absorbing multi-layered film containing at least one layer of a light transmitting nitride insulation film. The reflectance can be reduced to 5% or less by using a laminate comprising a silicon nitride film, a titanium nitride film and a silicon-nitride film. Similar effect can also be obtained by using a brown or black metal film such as a tantalum nitride film instead of titanium nitride.
Further, in each of the constitutions described above, other film partially absorbing light may be a film containing, at least one layer comprising Al, Cu, Au, Mo, Ni, Pt, Rh, Ag, W, Cr, Co, Si, Zr, Ta, INCONEL or NICHROME.
Further, each of the constitutions described above provides a light emission device in which the layer containing the organic compound is made of a material that emits white light and combined with a color filter disposed to a sealant or a light emission device in which the layer containing the organic compound is made of a material that emits a monochromatic light combined with a color conversion layer or a coloring layer disposed to the sealant.
Further, in accordance with the invention, after forming the concave shape of the first electrode, auxiliary wirings (also referred to as auxiliary wirings or third electrode) by vapor deposition using a vapor deposition mask may be formed on the insulator disposed between each of the pixel electrodes to lower the film resistance of the second electrode as a cathode (light transmitting electrode). Further, it is also a feature of the invention to form lead wirings by using the auxiliary wirings and conduct connection with other wirings present in the lower layer.
Further, the constitution of the invention for attaining each of the constitutions 1, 2, 3 and 4 above provides a method of manufacturing a light emission device comprising an anode, a layer containing an organic compound adjacent with the anode, and a cathode adjacent with the layer containing the organic compound, the method comprising the steps of
forming a light absorbing multi-layered film continuously without exposure to atmospheric air,
forming an insulator covering the end of a first electrode comprising the metal film,
conducting etching using the insulator as a mask to reduce the thickness at the central portion of the first electrode such that an inclined surface is exposed in the first electrode,
forming a film containing an organic compound, and
forming a second electrode comprising a light transmitting thin film on the film containing the organic compound.
In the constitution of the manufacturing method described above, a laminate comprising a light reflecting metal film and a metal film as an etching stopper are used in which the light reflecting metal film is etched and the light reflecting metal material is exposed to the inclined surface.
In the constitution of the manufacturing method described above, the first electrode is an anode and comprises a metal layer having a larger work function than the second electrode.
In the constitution of the manufacturing method, the step of continuously forming the second electrode and the light absorbing multi-layered film is conducted by a sputtering method.
In the constitution of the manufacturing method described above, the first electrode is a cathode and comprises a metal layer having a smaller work function than the second electrode.
Further, in the constitution of the manufacturing method described above, the insulator covering the end of the first electrode has a curved surface with a radius of curvature at the upper end, and the radius of curvature is 0.2 μm to 3 μm.
The EL element has a layer containing an organic compound capable of obtaining luminescence generated by application of an, electric field (Electro Luminescence), an anode and a cathode. The luminescence in the organic compound includes light emission upon recovery from a singlet excited state to the ground state (fluorescence), and light emission upon recovery from a triplet excited state to the ground state (phosphorescence). The light emission device manufactured by the manufacturing device and the deposition method according to the invention is applicable to both of the emission forms.
The light emission element having a layer containing an organic compound has a structure in which the layer containing the organic compound is sandwiched between a pair of electrodes, and the layer containing the organic compound usually comprises a laminate structure. Typical structure includes a laminate structure of “hole transportation layer/light emission layer/electron transportation layer” proposed by Tang, et al. of Kodak Eastman Company. The structure shows an extremely high emission efficiency, and most of light emission devices under research and development at present adopt the structure.
Further, structures formed by laminating, on the anode, hole injection layer/hole transportation layer/light emission layer/electron transportation layer, or hole injection layer/hole transportation layer/light emission layer/electron transportation layer/electron injection layer in this order may also be used. Fluorescent dye, etc. may be doped to the light emission layer. Further, all of the layers described above may be formed by using low molecular weight material or by using high molecular weight material. In the present specification, all the layers disposed between the cathode and the anode are collectively referred to as the layer containing the organic compound. Accordingly, all of the hole injection layer, hole transportation layer, light emission layer, electron transportation layer and electron injection layer are included in the layer containing the organic compound.
Further, in the light emission device of the invention, there is no particular restriction on the driving method of screen display and successive spot driving method or successive line driving method or successive plane driving method, etc. may be used. Typically, the successive line driving method may be used while properly adopting time divisional graduation driving method or area gradation driving method. Further, video signals inputted to the source line of the light emission device may be analog signals or digital signals and driving circuits and the like may be designed while properly adapted for video signals.